Radu Molasar in general. The living embodiment of all evil and even worse than the Nazis, he stands as a particularly nightmarish creation of the 80s that has that much power. The main reason to be afraid of him, for us, is his imposing appearance. And those eyes...
In both movie and book, Molasar does ghastly things to kill the German soldiers. The background of the titular keep, with the generator and lights sputtering on and off, only drives the viewer's unease higher.
Even on the non-supernatural side, there's Kaempffer — much worse here than in the original book — a cold, sociopathic Nazi who matter-of-factly has three completely innocent people murdered by his men the moment he steps into the village by the Keep and calmly informing that for every ensuing death caused by Molasar, five more will die until the SS runs out of innocents to murder. Even with Molasar's presence, Kaempffer stands out for being an utterly inhuman monster whilst still being completely human, unlike the demonic Molasar.
The final scene when Kaempffer walks into the main outer hall and sees his company burnt to a crisp, vehicles and gear included as flesh and steel is molten together by Molasar's power. The fact that it isnt show "how" it happens further emphasizes the horror of it as we are left to imagine it all, much to Kaempffer's personal horror as he stumbles in shock through the mass grave.
